Corona effect is an electrical phenomenon occurring in conductors which transport high voltage electric currents and appearing around the conductors in the form of a light halo which can sometimes be seen by the human eye. Given that conductors tend to have a circular section, the halo adopts a corona shape, hence the name of the phenomenon.
Corona effect is caused by ionisation of air surrounding the conductor due to the high voltage levels of the line. When the air molecules are ionised, they are capable of conducting electric current and part of the electrons flowing through the line will flow through the air. Such flow will cause a temperature increase in the gas which will turn reddish for low temperature levels, or bluish for high levels.
In the scope of the art, the voltage at which the field on the surface of the conductor exceeds the dielectric strength of the air and the corona effect begins is called disruptive critical voltage; and that greater than the disruptive critical voltage from which the corona effect is visible is called visual critical voltage.
Power loss caused by the electrical discharge from the conductors into the air in the installations is a direct consequence of this effect. Another particularly significant consequence is material deterioration, increasing the maintenance cost of the installations.
It is known that in the components of an installation subjected to high electrical voltage, such as the suspension or securing points of the aerial conductors, also called buses in an electrical sub-station, when these have surfaces which are not perfectly smooth or have pointed areas or edge, the corona effect acquires unacceptable levels.
The problems associated with these electrical discharges in the components of a conversion plant or sub-station are conventionally solved by means of using an anti-corona shield the purpose of which is to redistribute the electromagnetic field lines around the components through a surface with relatively high curvature, particularly close to the critical regions including edges or pointed areas.
These curved surfaces are formed by thin sheets of aluminium or other conducting materials with different shapes and which in pairs form, when they are assembled, closed toroidal- or spherical-shaped bodies or semi-open bodies having spherical cap shape or the like.
Sometimes stamping techniques are not suitable to provide the starting sheets with the required shapes and more so when smooth finishes without recesses or protrusions altering the desired curve shape are required, and the use of moulds and stamping is thus resorted to, the manufacturing costs becoming significantly more expensive.
The objective of the present invention is to disclose an alternative to the known anti-corona shields forming bodies partially or completely surrounding the components of the installation from one or several metal sheets.
Patent document WO2011/009482 describes a toroid-shaped shield as a possible solution for reducing costs. This solution consists of using an array of elongate tubular elements placed parallel to each another and attached to each other by means of annular structural elements they form a torus-like body but the walls of which are not continuous but are discrete or broken.
This construction allows lightening material, reducing costs and weight, however it is not satisfactory from the electrical behaviour viewpoint because the field lines are not satisfactorily redistributed largely due to the fact that outer surface of the shield is not continuous. To overcome this drawback a joint solution is proposed, combining the elongate tubular elements with metal sheets to form a surface which is a continuous surface in some sections and a discrete or broken surface in other sections.
Another objective of the present invention is a device for minimising corona effect which, in the line with that proposed in WO2011/009482, allows reducing the use of material but improving the performance thereof, inasmuch as the distribution of the current lines and the use of metal sheets which entails an additional cost in obtaining complex curved shapes or shapes of revolution.